164 C. M. CHILD. 



not occur normally, e. g., in animals reduced by starvation from 

 a length of 15— 18 mm. to a length of 6-7 mm., in young animals 

 of J-^ mm. in length, in the pieces resulting from the fission of 

 large individuals, and again in the pieces resulting from the fission 

 of pieces derived from fission; 



It follows from these facts that the posterior end of the first 

 zooid lies somewhere near the middle of the postpharyngeal 

 region in large indiyiduals, and posterior to this level is a second 

 young individual. 



III. The General Effects of the Anesthetics. 



In the first place the regulatory processes are much retarded 

 and the retarding effects are greater in small than in large pieces. 

 The most favorable cases in the anaesthetics require at ordinary 

 temperatures ten days or more to reach a stage which similar 

 pieces in water attain in four or five days. 



In a short paper ^ attention has already been called to the fact 

 that the change in the proportions of the pieces is retarded to a 

 much greater extent than the formation of new parts. As a 

 matter of fact the increase in length and the decrease in width of 

 the pieces, which is so characteristic a feature of the regulation 

 in Planaria does not occur to any appreciable extent until the 

 pieces have become sufficiently acclimated and have reached a 

 sufficiently advanced stage of development to move about in a 

 manner approaching the normal. In the paper above referred to 

 I pointed out that this fact confirms the conclusions which I have 

 already stated in previous papers, viz., that this change in pro- 

 portion is largely the result of mechanical conditions connected 

 with movement and locomotion. 



In general the formation of the new tail is retarded to a much 

 greater extent than that of the head. A small amount of new 

 tissue forms on the posterior cut surface, but outgrowth to form 

 the typical, tapering posterior end does not occur until the speci- 

 mens begin to creep about. Apparently the movement and use 

 of the posterior end is not only a factor in determining its shape, 

 but also stimulates the growth of new tissue. 



' Child, "The Regulatory Change of Shape in Planaria dorotocephala,'" BiOL. 

 Bum.., XVI., 6, 1909. 



I 



